That is caused by something, somewhere in your site, declaring a class called "InputFilter". This causes a conflict with the version that is installed with the calendar.

My short answer is that you will not be able to use the calendar.

My long answer is that you will need to figure out how the InputFilter class, that came with your PHP-Nuke, works and then compare that with the one that comes with the calendar. If they are compatible, then you should be able to empty out the class.inputfilter_clean.php file, or rather remove the part that declares the class...

In all of the times that I've integrated the calendar in PHP-Nuke, I've never encountered this. I've also never seen a theme that had so many validation errors (ie: you have 10 <html> tags, where a valid page is only allowed one).